1. Field of the Invention
The present invention relates generally to a multi-antenna system, and in particular, to an apparatus and method for removing interference in a transmitting end of the multi-antenna system.
2. Description of the Related Art
A multi-user multi-antenna system has conventionally employed either a Zero-Forcing (ZF) scheme or a Minimum Mean Square Error (MMSE) scheme. In the ZF scheme, a signal transmitted from a transmitting end is multiplied by an inverse of a channel matrix so as to reduce interference with a different user or a different antenna. In the MMSE scheme, signal transmission is achieved taking a noise variation of a channel into account.
The ZF scheme and the MMSE scheme are based on linear pre-coding and have advantages in that a transmission process of the transmitting end can be easily implemented, and an error rate is not significantly increased even when an amount of channel feedback information transmitted from users is not sufficient.
On the other hand, some examples of schemes based on nonlinear pre-coding include a Tomlinson-Harashima Precoding (THP) scheme in which Dirty Paper Coding (DPC) is applied to a one-dimensional vector and a Vector Perturbation (VP) scheme in which the DPC is applied to an nth dimensional vector. In such nonlinear pre-coding schemes, the transmitting end allows a Transmit (Tx) signal to be subject to a modulo operation so that a positive integer value is added to or subtracted from the Tx signal, and transmits the resultant signal to a receiving end. Even when the receiving end does not know the positive integer value, the receiving end can estimate the signal through the same modulo operation used in the transmitting end. Accordingly, the transmitting end can optimize both a channel and the Tx signal. Hence, the nonlinear pre-coding schemes have been researched as a promising technology in a Time Division Duplex (TDD) nomadic environment where feedback is frequently made to the transmitting end.
Meanwhile, the ZF scheme and the MMSE scheme have disadvantages, such as that performance deterioration and Tx power loss are inevitable, Tx power has to be constant for each user or each antenna, each user has to use only one antenna, and in particular, discrepancy between sum capacity and ideal capacity becomes significant as Signal-to-Noise Ratio (SNR) increases.
Moreover, the DPC-based nonlinear pre-coding schemes have demerits, such as that a data rate has to be constant for each user, and each user has to use only one Receive (Rx) antenna. Therefore, disadvantageously, Quality of Service (QoS) for each user cannot be properly ensured. In addition, complexity increases significantly since the transmitting end has to use an encoder based on complex Maximum Likelihood (ML) estimation.
Accordingly, there is a demand for a method in which performance can be maximized by optimizing a data rate and Tx power for each user while reducing complexity in a multi-antenna system.